1. Technical Field
The present invention relates to a double-tuned circuit used for a tuner such as a television receiver.
2. Related Art
There is known a double-tuned circuit of a television tuner in which a trap circuit for attenuating an image signal is formed. FIG. 9 is a circuit diagram illustrating an RF double-tuned circuit having an image trap formed therein. A television reception signal is input to a high frequency amplifier AMP1 through an antenna tuned circuit (not shown), and an output of the high frequency amplifier AMP1 is input to a primary side tuned circuit 1 of the RF double-turned circuit. A secondary side tuned circuit 2 is coupled to the primary side tuned circuit 1, a reception signal taken from the RF double-tuned circuit including the primary side tuned circuit 1 and the secondary side tuned circuit 2 is frequency-converted in a mixer circuit MIX1, and the converted signal is output to a decoding circuit (not shown)
A series circuit including an inductor L1 and a resistor R4 is connected between an output terminal of the high frequency amplifier AMP1 and a ground. One end of a resistor R3 is connected to a connection point between the output terminal of the high frequency amplifier AMP1 and the inductor L1, and the input terminal of the primary side tuned circuit 1 is connected to the other end of the resistor R3 through a capacitor C2. The ground side of the resistor R4 is connected to a direct-current blocking capacitor C3, and a direct-current voltage +B applied to a connection point between the capacitor C3 and the resistor R4 is applied to the high frequency amplifier AMP1.
In the primary side tuned circuit 1, a parallel connection circuit including a varactor diode D1 and a tuning coil L2 is connected between the input end of the primary side tuned circuit 1 and the ground. A cathode of the varactor diode D1 is connected to the ground through a capacitor C4. In the secondary side tuned circuit 2, a parallel connection circuit including a varactor diode D2 and a tuning coil L3 is connected between the output end of the secondary side tuned circuit 2 and the ground. A cathode of the varactor diode D2 is connected to the ground through a capacitor C5. A tuning voltage Tu is applied to the cathode of the varactor diode D1 of the primary side tuned circuit 1 through a resistor R5, and is applied to the cathode of the varactor diode D2 of the secondary side tuned circuit 2 through a resistor R6. The tuning frequency from the outside can be varied by controlling the tuning voltage Tu.
The output end of the secondary side tuned circuit 2 is connected to an input terminal of the mixer circuit MIX1 through a capacitor C6. A conductive line 3 extends from the output end side of the secondary side tuned circuit 2 to the primary side tuned circuit 1 side and a trap for attenuating an image signal frequency is formed in a very small capacitor Co formed between the conductive line 3 and a hot side of the tuning coil L2 of the primary side tuned circuit 1. The end of the conductive line 3 is connected to the input end of the high frequency amplifier AMP1 through the resistor R1. A connection point between the resistors R1 and R2 interpolated in the way of the conductive line 3 is connected to the ground through the capacitor C1.
FIG. 10 is a diagram illustrating the disposition of a coil mounting surface of the known RF double-tuned circuit. Air-cored coils formed in an air-cored shape are used as tuning coils L2 and L3 of primary side and secondary side tuned circuits 1 and 2. A pair of coil inserting holes are provided on a circuit substrate SUB at a predetermined interval, and the tuning coils L2 and L3 are inserted into and fixed to the coil inserting holes, thereby mounting the tuning coil L3 of the secondary side tuned circuit 2 formed in the air-cored shape to be concentric with the tuning coil L2 of the primary side tuned circuit 1 formed in the air-cored shape.
The tuning coils L2 and L3 are mounted and fixed onto the circuit substrate SUB, an adjusting worker directly inserts an adjusting rod or the like into the tuning coils L2 and L3 of the primary side and secondary side tuned circuits 1 and 2 and moves the adjusting rod or the like to widen or narrow the tuning coils L2 and L3, and the adjusting worker changes and adjusts the inductance values of the tuning coils L2 and L3 so as to take the inductive coupling necessary between the tuning coils L2 and L3 of the primary side and secondary side tuned circuits 1 and 2 and to obtain the desired tuning frequency of the primary side and secondary side tuned circuits 1 and 2. Two adjustments of taking the inductive coupling and adjusting the tuning frequency of each of the tuning circuits 1 and 2 are performed together by the tuning coils L2 and L3 of the primary side and secondary side tuned circuits 1 and 2.
An example of the related art is described in Japanese Patent Application Laid-Open No. 8-70236.
In a double-tuned circuit for tuning to a relatively high frequency such as a UHF band, the inductances of tuning coils L2 and L3 of primary side and secondary side tuned circuits 1 and 2 become small. Accordingly, to take the necessary inductive coupling, the tuning coils L2 and L3 of the primary side and secondary side tuned circuits 1 and 2 have to get very close.
However, it is difficult to make both coils close enough to obtain sufficient coupling due to the restriction of the coil inserting holes on the substrate. For this reason, it is necessary to adjust the tuning coils L2 and L3 fixed to the coil inserting holes so as to bring them closer, work efficiency is low, and there is a productivity problem.
In addition, such a double-tuned circuit has an influence on the frequency precision of an image trap formed in the tuned circuit, and thus it is difficult to obtain a uniform disturbance blocking characteristic within a reception band.